The useful output power P or energy E (E=Pτ, τ is the duration of the optical pulse in a pulsed system) from fiber amplifiers can be limited by nonlinear optical interactions in the fiber, such as self-phase modulation (SPM), cross-phase modulation (XPM), four-wave mixing (FWM), stimulated Raman scattering (SRS), and stimulated Brillouin scattering (SBS). These nonlinear interactions become detrimental when the intensity I (I=P/A, A is the effective cross-sectional area of the optical mode in the fiber) of the light propagating in the fiber reaches a threshold value. Standard single-mode fibers normally have an effective mode area below 100 μm2. Large-mode-area (LMA) fibers can have an effective mode area that reaches hundreds or even thousands of μm2. The large mode area of LMA fibers maintains a relatively low intensity I within the fibers while increasing the total output power P or energy E. Therefore, using LMA fibers can raise the threshold for the detrimental nonlinear optical interactions that limit the useful output power of an amplifier.
For several applications, it is desirable to have fundamental-transverse-mode output from fiber amplifiers. Techniques have been developed to operate LMA fiber amplifiers in the fundamental transverse mode despite the fact that the LMA fibers may support the propagation and amplification of higher-order modes. These techniques may breakdown when LMA fiber amplifiers are operated at high power. Specifically, the useful output power or energy from LMA fiber amplifiers can be limited by multimode instability (MMI), also referred to as transverse-mode instability (TMI) and sometimes just mode instability (MI), which couples power (energy) between the fundamental and higher-order transverse modes of the LMA fiber.